PRINCIPAL RESULTS (continued)Site 1165: Continental Rise (Wild Drift)
Prior to drilling, a single seismic-reflection profile was recorded across the location of Site 1165, using the ship's water-gun and digital recording system, to verify the location of the site. Three holes were drilled at the site. Hole 1165A consisted of a mudline core that was dedicated to high-resolution interstitial water sampling. Hole 1165B was cored with the APC to 147.9 mbsf (86.4% recovery) and deepened with the XCB to 682.2 mbsf (57.3% recovery). Hole 1165C was washed down to a depth of 54 mbsf, where a single core was taken at an interval that had been missed in Hole 1165B. Continuous RCB coring began at 673 mbsf and continued to a total depth of 999.1 mbsf, with 80% recovery. Coring operations were interrupted by five icebergs, which came to within 200 m of the drill site and caused a total loss of 98 hr. Hole 1165C was successfully logged with the triple-combo tool string from 176 to 991 mbsf and from 176 to 580 mbsf with the sonic tool.
Drilling at Site 1165 yielded a relatively continuous 999-m-thick sedimentary section of early Miocene- to Pleistocene-age terrigenous and hemipelagic deposits (Fig. 23) with only few minor (<2 m.y.) disconformities. Dispersed clasts (IRD) are present down to the bottom of the hole, but lonestones are infrequent below 500 mbsf (lower Miocene). They both are relatively abundant above 300 mbsf (middle Miocene).
The sedimentary section is divided into three lithostratigraphic units that are characterized by cyclic variations between biogenic-bearing (lighter) and terrigenous-dominated intervals (darker). The cyclic variations in lithology are also recorded as visual color alternations, and cycles in spectrophotometer lightness factor, bulk density, magnetic susceptibility, and other laboratory and downhole log parameters to varying degrees. In general, cores get darker downhole as biogenic intervals become thinner relative to the thickness of terrigenous-bearing intervals. For the same reason, light-dark cyclicity is more prominent above ~400 mbsf. Darker units generally have higher bulk density, magnetic susceptibility, and organic carbon values. The sediment consists mostly of quartz, calcite, plagioclase, K-feldspar, and a mixture of clay minerals, as well as minor hornblende and pyrite. Silt-sized components are mainly quartz, but plagioclase, biotite, amphibole, and other heavy minerals are common.
Unit I (0-63.8 mbsf) consists of structureless brown diatom-bearing clay and clay. There are beds with minor diatom-bearing greenish gray clay that have dispersed sand grains, granules, and lonestones. There is minor laminated silt and minor brown foraminifer-bearing clay. Foraminifers comprise 5%-15% of sediment in the upper 13 mbsf. One interval within Unit I (20?-30? mbsf) is characterized by alternations between two facies like those of Unit II (i.e., Facies II-1 and II-2).
Unit II (63.8-307.8 mbsf) is characterized by alternations of two main facies, which differ in color and composition. Facies II-1 consists of structureless, homogeneous, greenish gray diatom clay, and Facies II-2 is mostly dark gray diatom-bearing clay with some intervals of scattered silt laminae. Many lower boundaries of Facies II-2 are sharp, and upper boundaries are transitional with bioturbation that increases upward into Facies II-1. Higher amounts of siliceous microfossils and ice-rafted debris (floating sand grains and pebbles) are found in Facies II-1 than II-2. Facies II-1 is characterized by lower grain density because of the higher diatom content. A third facies (II-3) is sometimes found and consists of several 15- to 40-cm-thick nannofossil chalk beds that have a sharp base and pass gradually up into Facies II-1. Within Unit II, three subunits are identified based on the different proportions of Facies II-1 and II-2. Subunit boundaries are at 160 and 252 mbsf and partially denote amounts of lonestones, with greater amounts of lonestones in Subunits IIA and IIC than in Subunit IIB.
Unit III (307.8-999.1 mbsf) comprises a section of thinly bedded planar-laminated claystone that is divided, like Unit II, into two main facies that differ in color, composition, and bedding characteristics. Facies III-1 consists of greenish gray, bioturbated, structureless clay and claystone and diatom-bearing clay and claystone with dispersed coarse sand grains and rare granule to pebble-sized lonestones. Facies III-2 is composed of dark gray, thinly bedded, planar-laminated clay and claystone with abundant silt laminae. Lonestone (dolerite, diorite gneiss, and mudstone) abundance in Unit III is low and decreases downhole.
In Facies III-1, the thickness of the greenish gray intervals is generally less than 1 m, and some intervals have higher concentrations of material as coarse as sand sized. The upper contacts are commonly sharp, laminae are rare, and bioturbation increases upward. Angular mud clasts and benthic foraminifers are present in this facies below 800 mbsf. Siliceous microfossil content is low.
Facies III-2 becomes increasingly fissile with depth and changes to very dark gray or black below 894 mbsf. Bioturbation is rare in Facies III-2, and light-color silt laminae are a conspicuous feature (average = 150-200 laminae/m). Many cross-laminated silt ripples are present, and ripples are more common below 673 mbsf than above. Microfossils are rare and seem to disappear completely below ~600 mbsf. Below 842 mbsf, laminae with calcite cement are present and sections of the core become increasingly cemented with likely authigenic carbonates. A change to darker-color claystones occurs at 894 mbsf, where fracture patterns also become curved.
In both Facies III-1 and -2, individual 0.5-cm-sized horizontal burrows (Zoophycos) are evident along with clusters of millimeter-sized burrows.
An excellent record of siliceous microfossils is found at Site 1165 down to 600 mbsf, where biogenic opal disappears because of an opal-A/opal-CT diagenetic transition. Neogene high- latitude zonal schemes for both diatoms and radiolarians yielded identification of 21 diatom and 12 radiolarian biostratigraphic datums between 0 and 600 mbsf. Below 600 mbsf, age assignments are inferred from calcareous nannofossils, which are present in only a few discrete intervals with moderate to good preservation of assemblages that have low diversity. Nannofossils yield Pleistocene to lowermost Miocene ages. Benthic foraminifers are more common than planktonics, which are rare. The foraminifers indicate several intervals of redeposited material.
A magnetostratigraphy was determined for Site 1165 for the interval 0-94 mbsf and below 362 mbsf (Fig. 24). The magnetostratigraphic record and biostratigraphic ages, when combined, yield an age-versus-depth model that shows relatively rapid deposition in early Miocene time (~120 m/m.y.), somewhat slower deposition in middle to late Miocene time (~50 m/m.y.), and even slower deposition since late Miocene time (~15 m/m.y.) (Fig. 25). From magnetostratigraphy, the bottom of Hole 1165C (999.1 mbsf) has an age of ~21.8 Ma The uncertainty in ages is larger below 600 mbsf, where only few biostratigraphic ages exist to constrain the paleomagnetic reversal stratigraphy.
Measurements of rock magnetic properties in the interval 114-370 mbsf indicate that magnetic mineral concentrations (i.e., concentrations of magnetite) drop significantly, with nearly a complete loss of magnetic intensity. Shipboard analysis suggests that this unusual loss of magnetic signal is caused by diagenetic dissolution of magnetite in the presence of high silica-rich pore-water concentrations. A zone of low grain density is observed in cores from part of this zone ~140 mbsf and may also coincide with the disappearance of magnetite.
The interstitial water profiles document downhole sediment diagenesis at Site 1165. Sulfate values decrease linearly from 30 to 2 mM in the interval from 0 to 150 mbsf, but increases occur in ammonium (20 to 384 M), phosphate (3 to 10 M) and alkalinity (3 to 8 mM) throughout the same interval because of destruction of organic matter. Ammonium increases (to 800 M), phosphate decreases (to 0), and alkalinity decreases linearly (to 1 mM at 999 mbsf) throughout the interval from 150 to 400 mbsf. Concentrations of dissolved silica increase from 522 M at the seafloor to a maximum of 1000 M at 200 mbsf and reflect the dissolution of abundant siliceous microfossils. Silica values decrease from 400 to 999 mbsf. The theoretical opal-A/opal-CT transition is at ~600 mbsf, based on measured downhole temperatures. A strong seismic reflection is observed at this depth. Below 150 mbsf, calcium and magnesium values correlate inversely, which suggests diagenetic control and an unidentified Ca-rich lithology below the drilled section. A decrease in potassium values below 50 mbsf and an increase in fine-grained K-feldspar, identified by XRD measurements, suggests an authigenic origin for feldspars.
Hydrocarbon gas contents of cores are low (<400 ppmv) within the sulfate reduction zone down to ~150 mbsf (Fig. 26). Below 150 mbsf, methane increases rapidly and reaches values of 20,000 to 40,000 ppmv between 270 and 700 mbsf, and 40,000-100,000 ppmv from 700 to 970 mbsf. These headspace gas measurements indicate only the residual gas in the pore water of cores after outgassing upon retrieval to the surface. The gas concentrations are equivalent to ~8 to 35 millimoles per liter (mM) dissolved CH4 when adjusted for sample size variation, density, and porosity. Cores deeper than 700 mbsf may contain more gas because of increased lithification and retarded outgassing. Ethane is present in headspace gas samples deeper than 157 mbsf, and the C1/C2 value shows the expected decrease with depth for the observed geothermal gradient at this site (secant temperature gradient of 43.6°C/km). OC contents of Hole 1165 sediments are low (0.1 to 0.8 wt%), except for high organic carbon (1.8-2 and 1.3 wt%) beds at depths of 122 and 828 mbsf (Fig. 26). Cores from within the gas hydrate stability zone (seafloor to 460 mbsf) were examined immediately upon recovery, but no hydrates were observed.
Acoustic velocities and shear-strength measurements increase as a result of normal compaction (in the upper 500 mbsf) and diagenesis (with a greater effect below 600 mbsf). Horizontal P-wave velocities increase abruptly at 114 mbsf from ~1535 to 1575 m/s and then linearly from 114 to 603 mbsf. Other significant P-wave velocity increases occur at 692 mbsf, at 800 mbsf (to 2660 m/s), and 960 mbsf (to 2840 m/s). These abrupt increases are observed as reflections on seismic data.
Hole 1165C was logged with a single pass of the triple combo tool from 176 to 994 mbsf and a single pass of the sonic tool from 176 to 580 mbsf. Total gamma-ray and bulk-density log traces covary, suggesting that diatom-bearing strata (lower density) are present throughout the hole. The boundary between lithostratigraphic Units II and III at 305 mbsf is marked by a large fluctuation in total gamma-ray values, which suggests a shift in clay content at the boundary. The distinct silica- and calcite-cemented intervals observed in the cores are marked by peaks in most log traces.
Site 1165 provides paleontologic and sedimentologic evidence that numerous alternations or cycles between biogenic material and clay-sized terrigenous debris from Antarctica have occurred since earliest Miocene time (Fig. 27). The cyclicity observed in the lightness values for two intervals (83-100 mbsf and 107-123 mbsf; in lithostratigraphic Unit II) in the cores was analyzed for spectral content to evaluate the potential effect of Milankovitch periodicities on biogenic/terrigenous sedimentation cycles at this site (Fig. 27). For the shallower interval, significant spectral peaks are found at periods of 3.28, 1.45, 1.08, and 0.64 m thickness. A sedimentation rate of 3.5 cm/k.y. is reasonably well constrained for the interval and gives periods of 93.7, 41.5, 20.8, and 18.2 k.y., respectively. These periods are similar to the Milankovitch cycles of 100 k.y. (eccentricity), 41 k.y. (obliquity), 23 k.y. and 19 k.y. (precession), suggesting an orbitally forced origin for the light-dark cyclicity. For the deeper interval, significant spectral peaks are at 4.27, 1.55, 0.95, and 0.72 m, which have similar peak periodicity ratios to the upper interval, again suggesting an orbital origin for the deeper cycles. In the deeper interval, the sedimentation rate is less well constrained. But, by using the peak-to-peak ratios as a guide, an inferred sedimentation rate of 3.8 to 4.1 cm/k.y. would give the same periods as the shallow interval (i.e., 93.7, 41.5, 20.8, 18.2 k.y.). The observed lightness changes correlate also to variations in bulk density and magnetic susceptibility, indicating that the lightness (color) data likely document orbitally driven changes in the Site 1165 depositional environment since at least early Miocene time. A similar approach has been used recently to detect orbital signals in upper Oligocene to lower Miocene sedimentary sequences drilled in the Ross Sea, Antarctica, by the Cape Roberts Project (Claps et al., in press). Milankovitch cyclicities have been reported for late Miocene-age and younger diatom-bearing hemipelagic and terrigenous sediments at ODP Site 1095 from a drift deposit, like the Wild Drift, adjacent to the Antarctic Peninsula (Shipboard Scientific Party, 1999).
In addition to the cyclic variations, significant uphole changes occurred at Site 1165 from early to late Miocene times (Units III and II), and include an eightfold decrease in average sedimentation rates from 12 to 1.5 cm/k.y., an increase in the total clay content, an increase in amount of sand-sized and lonestone IRD, and other changes (e.g., first appearance of glauconite) (Fig. 22, Fig. 25). The changes likely reflect shifts, in the onshore Prydz Bay region (sediment source area), to paleoenvironments that produced less terrigenous sedimentation, a lower-energy current regime, and more floating ice at Site 1165 starting in middle Miocene time. The uphole shift to increased clay (~305 mbsf) and first appearance of glauconite (~220 mbsf) heralds (1) a mid-Miocene change to erosion of sedimentary basins on the shelf and (2) subsequent inferred overdeepening of the shelf to the large water depths of today. Such deep-cut erosion would be by grounded glaciers crossing the continental shelf and dispersing icebergs with entrained sediment. The times of lowest sediment supply, in the latest Neogene (i.e., above 60 mbsf), have upward-decreasing silica contents (i.e., fewer diatoms and sponge spicules) and varied (cyclic?) IRD concentrations, which may reflect increasing extent of sea ice cover and ice sheet fluctuations. During latest Neogene time, a widely recognized period of intense Antarctic (and Arctic) glacier fluctuations, thick debris flows blanketed the adjacent continental slope (Site 1167), but little sediment was deposited on Wild Drift (Site 1165).
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